This invention relates to locating and aligning the spine of a golf club shaft. More particularly, this invention relates to a method and apparatus for automatically and reliably identifying the location of the spine of a golf club shaft and for aligning the spine in a desired orientation.
When a golfer swings a golf club, the shaft of the golf club bends or twists, especially during the downswing. The direction the shaft bends or twists is dependent on how the golfer loads or accelerates the club, but the bending or twisting direction and magnitude also are dependent on the stiffness of the shaft. If a shaft is soft, it will bend or twist more during a given downswing than if it is stiff. Additionally if a shaft exhibits different transverse stiffness in different planesxe2x80x94i.e., the stiffness, roundness and straightness of the shaft are not symmetricxe2x80x94the shaft will bend or twist differently depending upon in which plane (direction) it is loaded.
Immediately prior to the impact of the head of a golf club with a golf ball, the shaft of the golf club goes through significant vibratory movements in both the toe up/down direction (plane perpendicular to the hit direction) and in the lead/lag direction (plane parallel to the hit direction). Research has shown the shaft of a golf club vibrates up and down in the toe up/down direction immediately prior to impact with the golf ball. This up and down movement, known as xe2x80x9cvertical deflectionxe2x80x9d or xe2x80x9cdroop,xe2x80x9d can be as large as xc2x11.5 inch (xc2x13.8 cm). Because any inconsistent bending or twisting due to asymmetric shaft behavior immediately prior to impact is substantially impossible for the golfer to correct with his or her swing, any reduction in vertical deflection or droop immediately prior to impact will help the golfer improve his or her impact repeatability. This is true for golfers of all skill levels. Inconsistent bending or twisting makes it more difficult for the golfer to reproduce the downswing shaft bending or twisting from club to club, thereby resulting in less consistent impact repeatability within the set.
In addition, a golf club, immediately prior to impact, xe2x80x9cspringsxe2x80x9d forward in the direction of the shot. This is commonly referred to as the xe2x80x9ckickxe2x80x9d of the shaft. If it is possible to analyze and orient a shaft in a way so that the kick direction of vibration is stable, this shaft position would improve the golfer""s ability to repeat the impact position with the ball. In other words the shaft would have less of a tendency to xe2x80x9cbobxe2x80x9d up and down immediately prior to impact thereby improving impact repeatability.
Inconsistent bending or twisting contributes to movements of the club head that would not be present if the shaft had been perfectly symmetric. Golf club shaft manufacturers attempt to build shafts with symmetric stiffness to minimize inconsistent bending or twisting during the swing, but as a result of manufacturing limitations it is difficult to build a perfectly symmetric golf club shaft. Specifically, it is well known that, as a result of irregularities or variations in materials or manufacturing processes, golf club shafts have a preferred angular orientation or xe2x80x9cspine.xe2x80x9d (See, e.g., U.S. Pat. Nos. 4,958,834 and 5,040,279, which are hereby incorporated by reference in their entireties.) Therefore, substantially all golf club shafts exhibit some degree of asymmetry which results in some degree of inconsistent bending or twisting during the swing.
The asymmetric stiffness of golf club shafts can result from nonsymmetrical cross sections (shafts whose cross sections are not round or whose wall thicknesses are not uniform), shafts that are not straight, or shafts whose material properties vary around the circumference of the shaft cross section. Because it is substantially impossible to build a perfectly symmetric golf club shaft and the objective is to minimize inconsistencies from club to club in a golf club set and from set to set within a brand, it makes sense, if possible, to analyze each golf club shaft in a set of golf clubs to understand its asymmetric bending or twisting behavior and construct the golf clubs in the set to maximize consistency from club to club within a set and from set to set within a brand.
It has been recognizedxe2x80x94e.g., in above-incorporated U.S. Pat. No. 5,040,279xe2x80x94that although substantially all golf club shafts exhibit some degree of asymmetry, substantially every golf club shaft exhibits at least one orientation in which, when the shaft is clamped at its proximal, or handle, end and displaced at the tip, the resultant vibration of the shaft will remain substantially planar. That is, the shaft will remain substantially in a single plane and the tip of the shaft will vibrate back and forth substantially along a line.
It is also recognized in above-incorporated U.S. Pat. No. 4,958,834 that the construction of all golf clubs within a set with their respective planar oscillation planes (xe2x80x9cPOPsxe2x80x9d) oriented in the same angular direction relative to their respective club faces will exhibit less inconsistency in shaft bending or twisting during the downswing than a set that has been haphazardly or randomly constructed. In particular, a set of golf clubs normally will function best if the respective preferred angular orientations of the respective golf club shafts are aligned in the xe2x80x9chit directionxe2x80x9dxe2x80x94i.e., substantially perpendicularly to the respective golf club faces.
However, heretofore there has not been any convenient automated way to determine the preferred angular orientation of a golf club shaft. It would be desirable to be able to provide a method and apparatus for quickly and reliably determining the preferred angular orientation of a golf club shaft. It also would be desirable to be able to provide a method and apparatus for using the determination of the preferred angular orientation to automatically assemble golf clubs with each respective golf club shaft consistently aligned relative to the respective club face.
It is an object of this invention to attempt to provide a method and apparatus for quickly and reliably determining the preferred angular orientation of a golf club shaft.
It is also an object of this invention to attempt to provide a method and apparatus for using the determination of the preferred angular orientationxe2x80x94e.g., the planar oscillation planexe2x80x94to automatically assemble golf clubs with each respective golf club shaft consistently aligned relative to the respective club face.
In accordance with the present invention, there is provided a method of determining a preferred angular orientation of a golf club shaft about a longitudinal axis thereof, where the golf club shaft has a proximal end for gripping by a golfer and a distal end for attachment to a golf club head. According to the method, the proximal end of said golf club shaft is immobilized, and vibratory motion of the distal end of the golf club shaft is initiated in a direction other than parallel to the longitudinal axis. The vibratory motion is analyzed, and from the analyzed vibratory motion he preferred angular orientation is calculated. The golf club shaft can then be marked to indicate the preferred angular orientation. In a further method according to the invention, the mark on the shaft indicating the preferred angular orientation can be used to automatically assemble a golf club with the golf club shaft in a predetermined alignment relative to the face of the golf club head.
Apparatus for determining the preferred angular orientation, and for assembling golf clubs, are also provided.